LD50 and Draize Tests
The LD50 test, invented in 1927, determines the dose that kills 50% of a group of 60-100 animals. The Draize eye test, developed in the 1940s, applies 0.1 mL of a substance to the eye of a conscious animal for 72 hours. Both have been criticized as cruel and scientifically unreliable. OECD began phasing out the classical LD50 in the 1980s-90s, but approximately 11,500 LD50-type procedures were still conducted in the UK alone in 2023.
The LD50 Test
The LD50 — Lethal Dose 50% — was invented in 1927 by J.W. Trevan. The protocol is straightforward: administer escalating doses of a substance to groups of animals until half of them die. A standard test uses 60 to 100 animals. Death is the measured endpoint. The dose at which 50% die becomes the LD50 value, used to classify the substance's acute toxicity.
The test was designed for an era when toxicology had few tools. It remains embedded in regulatory frameworks despite decades of criticism.
The Draize Test
The Draize eye irritation test was developed in the 1940s by John Draize at the U.S. Food and Drug Administration. The procedure involves placing 0.1 mL of a test substance into the eye of a conscious, restrained animal — historically rabbits, but dogs have also been used. The eye is observed for up to 72 hours. Scoring covers opacity, swelling, discharge, and tissue damage.
The Draize test has been criticized on both welfare and scientific grounds. Animals are not sedated during application. The test produces subjective, scorer-dependent results. Interspecies extrapolation is unreliable.
Phase-Out Efforts
The OECD began replacing the classical LD50 in the 1980s and 1990s with alternative protocols that use fewer animals: the Fixed Dose Procedure, the Acute Toxic Class Method, and the Up-and-Down Procedure. These reduce animal numbers but do not eliminate lethal endpoints.
Despite decades of phase-out language, approximately 11,500 LD50-type procedures were still recorded in UK data for 2023. Globally, the number is far higher but impossible to calculate precisely due to data and transparency gaps.
Partial Replacements
Several in vitro methods have been validated as partial replacements for the Draize test.
- BCOP — Bovine Corneal Opacity and Permeability assay, uses slaughterhouse-derived corneas
- ICE — Isolated Chicken Eye test
- FL assays — Fluorescein Leakage assays using cell cultures
These methods can identify severe irritants, reducing the need for animal tests at the extremes of the toxicity spectrum. They cannot yet fully replace in vivo testing for moderate irritants.
The Persistence Problem
Both the LD50 and Draize tests persist not because they are scientifically superior but because they are regulatory defaults. Changing a test guideline requires international consensus, validation studies, and regulatory acceptance — a process that takes years to decades. The tests are embedded in regulatory frameworks across jurisdictions that move at different speeds.
Sources
- 1.OECD Test Guidelines, various. Phase-out of classical LD50 and validation of alternative methods.
- 2.UK Home Office Statistics, 2023. Approximately 11,500 LD50-type procedures recorded.
- 3.Historical Toxicology Literature, 1927-1940s. Original LD50 (Trevan, 1927) and Draize (1944) publications.